Limited voltage capability of junction devices, such as diodes and transistors, is known, resulting from most of the applied voltage being confined to the very thin region on either side of the junction. This region is commonly referred to as the depletion region and is identified as the “space charge region” illustrated in FIG. 1. The thickness of this region is governed by many of the physics processes occurring in the device and can generally be only microns thick for most doping amounts. As a result, very little voltage divides across the bulk of the device (e.g., the thick vertical lines on the left extreme of the p-doped and the right extreme of the n-doped regions below). This effect is shown in the schematically represented voltage curve in FIG. 1. Thus the volume between the depletion region boundaries and the electrodes (labeled “neutral region”) are largely useless in increasing the amount of voltage that can be applied to the overall device. For junction devices made from GaN and SiC, this breakdown can occur in the range of 400 to 4,000 V across the device.
Wide bandgap semiconductor (WBS) materials are also known, e.g. GaN, SiC, etc., which when illuminated, irradiated, or otherwise energized by a radiation source of photons or subatomic particles, renders the material conductive, and may therefore be used, for example, as a photoconductive switch. Typically, the optical transport used to transport the radiation from the radiation source (e.g. a laser) to the WBS material is either through “free space” or an intermediate optical conduit, e.g. fiber optic cables, as illustrated in FIG. 2. In the case of optical conduits, however, at least two additional optical surfaces are required between the radiation source and the WBS: one additional surface (i.e. first optical conduit surface) forming the interface between the radiation source and the optical conduit, and another additional surface (i.e. second optical conduit surface) forming the interface between the optical conduit and the WBS. Each optical surface can contribute about 4% loss, and these loses, termed Fresnel losses, functionally follow a power multiplier. That is:Total Transmitted Energy=(Initial Energy)(1-fractional loss at each interface)Total Interfaces Thus for two interfaces as in the above example, only 92% of the initial energy from the optical source is transmitted to the substrate. Because the optical source can be highly inefficient (typically only about 10% efficiency in the best cases, but more typically 1% efficiency), any lost optical energy substantially impacts the overall system.
Furthermore, radiation sources such as optical sources have a certain amount of beam divergence and the acceptance of the optical conduit (e.g. fiber) is generally much less than that divergence. In addition, the divergence exiting the fiber is very large and some energy can be lost at the interface between the optical conduit and the WBS material due to this divergence. To overcome this loss requires the addition of lenses between the radiation source-to-optical conduit interface and the optical conduit-to-WBS interface. Unfortunately, each lens used introduces two interfaces per lens. So then in this example, only 78% of the initial energy from the optical source is transmitted to the substrate. While these loses can be reduced with special coatings at each interface, this technique can introduce added complexity and cost to the overall device. In addition to these losses, the added length of the optical conduit/fiber attenuates the optical energy resulting in more losses. And finally, the typical size of the output of the radiation source can be several square millimeters. And the WBS material can similarly be several square millimeters. The fiber, however, must be substantially less than those areas to maintain the mechanical flexibility. Thus, because the energy from the optical source must be concentrated into the fiber, the surface can be easily burned or damaged.
Electrically, the control/modulation of the WBS is functionally dependent on the capacitance of the overall device. But for a system, the desire is to minimize the number of optical feeds. Thus each switch must be fairly large in extent to minimize the number of optical feeds. As a result, each switch is highly capacitive. This capacitance adversely impacts the range in high frequencies that can be achieved. One method of minimizing this problem is to use a distributed capacitive system, such as shown in FIG. 3, having multiple optically triggered switches, in place of a lumped capacitive system having a single switch, in a circuit to drive a load (e.g. an antenna). The distributed capacitive system would produce a better frequency response due to reduced switch capacitance. Thus the useful range of frequencies can be extended by using a distributed system. However, one difficulty with such a distributed capacitive system is the complexity of multiple optical fibers being used to transmit the optical energy from the radiation source. For instance, a multiple fiber bundle would be required connected with specific outputs to connect the radiation source at point A to each switch so that the electrical delay from each of the distributed switches SA, SB, . . . and SC to point B is exactly compensates by the optical delay. The objective in such an arrangement is an equal delay from point A to point B through any switch SN, which can be problematic.
Additionally, in many wide bandgap fabrication methods, the desired WBS substrate (e.g. gallium nitride, GaN) is by necessity grown on a supporting substrate. In the particular case of GaN, the supporting substrate can be either sapphire or silicon carbide. With such processes, after the desired material is of sufficient thickness is grown, it must be removed from the sapphire or silicon carbide by invasive techniques. One such technique is a laser lift-off technique where the laser is applied through the transparent gallium nitride causing rapid heating at the gallium nitride and sapphire or silicon carbide interface. However, spalling and fracture may then occur at the boundary to enable the deposited gallium nitride to be removed. Such processes can generate defects and result in eventual failure of the gallium nitride under modulation conditions.